The present invention relates generally to control of the flow of refined metal in an ESR-CIG apparatus. The ESR apparatus is an electroslag refining apparatus and the CIG apparatus is a cold wall induction guide tube apparatus, also referred to herein as a cold wall induction guide mechanism and a cold finger nozzle mechanism. More particularly, the invention relates to controlling the superheat (temperature) of liquid metal flowing to, through and from (as a metal stream) the CIG apparatus. Most particularly, the invention relates to controlling the superheat of the metal provided to an atomization zone during spray forming operations by varying the superheat dynamically in coordination with an atomization manifold oscillation angle.
Such control of the liquid metal superheat is important to numerous applications which can be made using the refining apparatus including atomization processing and relates generally to direct processing of metal passing through an electroslag refining operation. One example of molten metal refining is referred to as electroslag refining, and is illustrated and described in U.S. Pat. No. 5,160,532--Benz et al, assigned to the same assignee as the present invention, the disclosure of which is hereby incorporated by reference.
In an electroslag process, a large ingot of a preferred metal may be effectively refined in a molten state to remove important impurities such as oxides and sulfides which may have been present in the ingot. Simply described, electroslag refining comprises positioning a metal ingot over a pool of molten material in a suitable vessel or furnace where the molten material pool may include a surface layer of solid slag, an adjacent underlayer of molten slag and a lowermost body of refined molten ingot metal. The ingot is connected as an electrode in an electrical circuit including the molten metal pool, a source of electrical power and the ingot. The ingot is brought into contact with the molten slag layer and an electrical current is caused to flow across the ingot/molten slag interface.
This arrangement and process provides electrical resistance heating of the slag and melting of the ingot at the noted interface with the molten ingot metal passing through the molten slag layer as a refining medium to become a part of the body of refined ingot metal. It is the combination of controlled resistance melting and passage of the molten ingot metal through the molten slag layer which refines the ingot metal to remove impurities such as oxides, sulfides, and other undesirable inclusions.
Spray forming is a process using gas atomization to produce a spray of droplets of liquid metal followed by solidification of the spray on a solid body to directly form a billet or billet preform. In metal spray forming, a small stream of refined molten metal from the furnace is directed to pass through a molten metal spray forming atomizer generally comprising a closed peripheral manifold about a central aperture. The manifold may be equipped with gas inlet means and plural gas jet exit means. A gas under pressure is supplied to the manifold to exit through the gas jets in converging streams which impinge the passing metal stream to convert or break up the metal stream into a generally expanding spray of small molten metal droplets. This spray is caused to impinge and deposit on a suitable collector surface to generate a metal billet or other metal object.
An important variable in this process is the gas-to-metal ratio (GMR) which indicates the amount of atomization gas relative to the amount of molten metal which is required to effectively atomize the metal stream to form a spray and to cool the spray in-flight before striking the billet or preform. The spray is scanned across a revolving substrate to build a uniform layer. As it becomes necessary to enlarge the diameter of the preform, it becomes increasingly necessary to control the local temperature of the spray. A relatively hotter spray is desired near the outer diameter of the preform, a relatively cooler spray is desired at the centerline of the preform.
Best results are believed obtained when the molten metal spray pattern from the atomization zone is directed angularly against the collector or preform object rather than perpendicular. An angular impingement provides improved deposition efficiency as well as improved preform metal density and microstructure.
Most previous attempts at varying the gas to metal ratio (GMR) targeted the variation of the gas pressure, thus varying the quantity of gas applied to the atomization process while maintaining the metal stream flow rate as near constant as possible. While this approach has been successful, such an approach is difficult to implement because the gas pressures must be rapidly pulsed.
An alternate approach has recently been suggested in copending patent applications Ser. No. 08/537,966 and Ser. No. 08/537,963 assigned to the assignee of the present application, the disclosure of each is hereby incorporated by reference. In these patent applications, the approach disclosed included methods and systems for varying the molten metal flow rate to the atomization zone while maintaining the rate of delivery of the atomizing gas to the molten metal stream constant thereby minimizing the gas pulsation control problem if not eliminating it altogether.
While this approach has the potential for significant cost savings, an alternate method and system for controlling the temperature of the spray impacting the preform would be to vary the temperature of the molten metal melt entering the atomization zone and thus the temperature impacting the preform during spray forming operations.
Thus, it would be desirable to develop systems for varying the superheat of the molten metal provided to the atomization zone while maintaining the rate of delivery of the atomizing gas to the molten metal stream constant in order to control the temperature of the metal spray delivered to the preform. Such systems could include, among other means, providing varying power to the CIG unit, including the induction power, voltage or current so as to vary, for example, electromagnetically or thermally, the superheat of the metal proximate exit orifice from the CIG, which would in turn dynamically vary the temperature of the metal melt flow therefrom to the atomizer and to further coordinate the controlled, varying metal superheat flow with the scan angle of the atomizer relative to the preform in order to achieve the appropriate spray temperature at various oscillation angles on contact with the preform.